This invention is directed to gyroscopes and, more particularly, to control systems for controlling the rate of rotation of the spinning mass (wheel) of a gyroscope motor such that the spin rate remains constant in inertial space.
Gyroscopes are widely used in navigation and control systems to provide information about the rate of movement of the vehicle with which they are associated about three orthogonal axes, normally referred to as the yaw, roll and pitch axes. Depending upon the accuracy required, gyros vary from relatively inexpensive, low accuracy mechanisms to relatively expensive, high-precision mechanisms. Regardless of their expense and accuracy, most presently available rate sensors include a spinning mass. The spinning mass, or wheel, of a rate gyro may be continuously torqued by a rebalance servo so that its spin axis is maintained fixed relative to fixed reference, such as a platform in a platform inertial guidance and navigation system or the vehicle in a strap-down inertial guidance and navigation system. In either system, when gyros are used in long-range navigation systems, such as the systems used on long-range aircraft, the servo's torquer scale factor must be extremely stable and precisely known. The required accuracy is a few parts per million. The torquer scale factor is inversely proportional to the spin speed (angular momentum) of the spinning mass of the gyro. In this regard, spin speed refers to the angular velocity of the gyro with respect to inertial space, not with respect to the vehicle. As a result, in order for the torquer scale factor to be extremely stable and precisely known, the rotational speed of the spinning mass in inertial space must either be known very accurately at all times or, preferably, held very constant at a known speed.
In a typical gyro, used in an aircraft inertial navigation system, for example, the gyro wheel or spinning mass is driven directly by a polyphase synchronous motor. The stator of the polyphase synchronous motor, which includes the armature windings, is mounted in the case of the gyro. In a conventional manner, the armature windings are excited by a very stable frequency source and set up a rotating field, which is followed by the rotor field. As a result, the gyro wheel rotates at the same speed as the speed of the rotating field created by the armature windings. (Of course, a load angle, .phi., exists between the rotor field and the rotating field of the armature winding.) Since the speed of the rotating field is relative to the armature windings, which are mounted in the gyro case, and since the rotating magnetic field speed is proportional to the frequency of the polyphase source, if the frequency of the polyphase source is maintained highly stable at a known value, the speed of the gyro wheel is maintained at a highly stable known value in inertial space. Of course, this result is only true if the gyro case and, thus, the armature windings do not rotate about the gyro spin axis. If the gyro case does rotate about the gyro spin axis owing to the movement of the vehicle, the speed of the gyro tends to remain constant with respect to the moving vehicle, not with respect to inertial space.
In platform inertial guidance and navigation systems, gyro case rotation about the gyro spin axis is prevented because the platform on which the gyros are mounted remains fixed in inertial space. This benefit, enjoyed by platform systems, does not apply to strapdown systems because such systems do not include a platform that remains fixed in inertial space. Rather, in a strapdown inertial guidance and navigation system, the cases of the gyros are attached to the body of the vehicle. As a result, the gyro spin axes are forced to follow the movement of the vehicle. Consequently, when the vehicle rotates about the spin axis of a particular gyro, the speed of the rotating armature field of the gyro no longer remains constant with respect to inertial space. Rather, the rotational motion of the vehicle about the spin axis of the gyro will either increase or decrease the speed of the armature field and, thus, the speed of the gyro wheel relative to inertial space.
Since vehicles move in a random fashion as a result of many external conditions, strapdown gyro cases rotate about their spin axes in a random manner. Because case movement is random, the instantaneous angular velocity of the rotating armature field varies in a random manner.
The foregoing undesirable feature (i.e., random variations in gyro wheel speed due to vehicular motion) of strapdown rate gyros using synchronous motors is compounded by the undesirable dynamic characteristics of the gyro motor. More specifically, when the gyro case displays any rotational motion about the gyro's spin axis, this motion excites the natural or hunting frequency of the synchronous motor. When this motion is at a frequency near the hunting frequency (usually one to five Hertz depending upon the type of gyro), the gyro load angle fluctuations become very large because the motor is lightly damped. The gyro load angle fluctuations further contribute to the deviation of the gyro wheel speed from a constant inertial angular velocity.
The worst effect of gyro wheel speed variations on the accuracy of inertial navigation is observed when the vehicle on which the gyro is mounted exhibits an oscillatory angular motion which has a vector component both along the gyro spin axis and along an input axis of the gyro. When such oscillatory angular motion occurs, the gyro experiences a slightly different scale factor in each half-cycle of the oscillation, resulting in a small unidirectional drift or bias effect. This unidirectional bias effect is sometimes referred to as "motor dynamics error". The magnitude of this error is too large to be tolerated in inertial navigation systems whose total gyro bias must be less than 0.01.degree./hour.
In the past, various proposals have been made to overcome and eliminate the errors created by movement of a gyro case about the spin axis of the gyro. These proposals can be grouped under two approaches. The first approach maintains the frequency of the gyro motor power supply constant and measures changes in inertial speed from a constant inertial speed. The second approach varies or modulates the frequency of the gyro motor power supply in such a way that variations in the rotational speed of the armature field caused by vehicle movement about the spin axis of the gyro are compensated. If compensation for such movement is done correctly, gyro wheel speed is not subject to angular accelerations and decelerations. In summary, the first approach measures speed errors and the resultant error measurements are used to modify the gyro output data in an associated computer. The second approach modifies armature excitation frequency so as to compensate for movement of the gyro case whereby the resulting output data do not include errors and, thus, do not have to be modified.
One suggestion for implementing the first approach described above comprises distributing small permanent magnets about the periphery of the rotor. The magnets induce pulses in a coil attached to the stator (gyro case) as the rotor turns. The time between pulses is measured in a highly accurate manner and the measurements are used to provide an indication of the angular velocity of the rotor with respect to the gyro case. The angular velocity of the rotor with respect to its case is then summed in an associated computer with the angular velocity of the gyro case with respect to inertial space to yield the actual wheel speed with respect to inertial space.
The problem with the foregoing proposal is that it is difficult to implement in practice. First, complex, time consuming computations are required. Second, it is not practical to obtain sufficient resolution and accuracy from such a pulse generating system, without using a prohibitively large number of pins and an extremely fast timing clock (above 20 MHz). Reference is hereby made to U.S Patent Application, Ser. No. 861,898, filed Dec. 19, 1977, entitled "Gyroscope Wheel Speed Modulator" by Guy R. Olbrechts for a more a detailed discussion of the foregoing implementation of the first approach described above and the reasons why it is unsatisfactory in most environments.
One suggestion for implementing the second approach described above is set forth in U.S. Patent Application, Ser. No. 861,898 referenced above. In this approach, the frequency of the polyphase power applied to the gyro is modulated in such a way that the rotating flux wave is maintained precisely constant with respect to inertial space, regardless of the rotation of the gyro case. The polyphase power is modulated in direct proportion to the angular velocity of the case, as measured by a second gyro. The proper amount of frequency modulation is applied in the proper sense to cancel the effect of gyro case rotation. Since the resulting flux wave travels at a constant inertial velocity, the hunting mode of the gyro system is not excited, whereby no fluctuations in load angle occur. Since this proposal compensates for fluctuations in case movement, the resulting gyro rate information does not have to be modified, as noted above. Reference is hereby made to U.S. Patent Application, Ser. No. 861,898 for a more detailed discussion of this implementation.
While the implementation described in U.S. Patent Application, Ser. No. 861,898 has been found to be entirely satisfactory in eliminating the effect of case movement on the rotational speed of a gyro mass, it has certain disadvantages. For example, each gyro depends upon another gyro for the measurement of the angular velocity of the vehicle about the first gyro's spin axis. This interdependency of gyros is disadvantageous in a navigation system because if the sensing gyro fails the dependent gyro information is erroneous. Secondly, the gain of the signal produced by the sensing gyro must be closely controlled (to about .+-.2%) in order to achieve precise cancellation of the effect of case rotation about the spin axis of the dependent gyro. If the batch spread in scale factor of either the frequency source (which is described in U.S. Patent Application, Ser. No. 861,898 as preferably taking the form of a voltage controlled crystal oscillator) or the sensing gyro is greater than .+-.2%, a calibration procedure is required to determine the gain that must be applied to the signal produced by the sensing gyro. Third, while the system described in Patent Application, Ser. No. 861,898 may include a phase-locked loop, it is essentially a means of providing an open loop compensation for the effect of vehicle movement. The inclusion of a phase-locked loop only preserves the long-term frequency stability of the voltage controlled crystal oscillator. Because it is essentially open loop, this sytem does not actually modify the natural or hunting frequency mode of the gyro. As will be readily appreciated by those skilled in the gyro art, the damping of this mode is normally very low (Q ranges from 20 to 100). As a result, any small phase shifts in the incoming angular rate signal make it difficult, if not impossible, to exactly compensate by adjusting the gain applied to the signal produced by the sensing gyro alone. Further, random noise in the bearing system (including g-sensitive bearing drag) tends to excite the hunting mode such that the rotor perpetually oscillates back and forth through a small angle. Fourth, the open loop nature of the system described in U.S. Patent Application, Ser. No. 861,898 requires that the voltage controlled crystal oscillator have a very linear voltage-to-frequency characteristic. Any non-linearity causes imperfect cancellation of case rotation at related amplitude levels. Because commercially available voltage controlled crystal oscillators, when specified to have a non-linearity of less than 1%, generally exhibit large frequency variations with temperature, the system described in U.S. Patent Application, Ser. No. 861,898 requires the inclusion of a phase-locked loop. In other words, in order to temperature stabilize the center frequency and maintain desired linearity relationships using presently commercially available voltage controlled crystal oscillators, it is necessary to include a phase-locked loop. Obviously, it would be desirable to provide a system for controlling the wheel speed of a gyroscope that does not require the inclusion of a precisely linear voltage controlled crystal oscillator. Moreover, it would be desirable to provide a controller for controlling the wheel speed of a gyroscope that does not require the inclusion of a phase-locked loop.
In general, it is an object of this invention to provide a new and improved controller for controlling the rotor speed of a synchronous motor.
It is a further object of this invention to provide a new and improved gyroscope wheel speed controller.
It is also an object of this invention to provide a closed-loop speed controller suitable for controlling the wheel speed of a gyroscope.
It is another object of this invention to provide a closed-loop gyroscope wheel speed controller that does not depend upon measurements made by another instrument, such as another gyroscope.
It is yet another object of this invention to provide a closed-loop gyroscope wheel speed controller that does not require the inclusion of a phase-locked loop.